The water present within an ionic liquid that is in a liquid state at 25.degree. C. or an organic solution containing at least one ionic compound is decomposed by bringing electrodes into contact with the ionic liquid or organic solution within an atmosphere having a dew-point temperature not higher than -40.degree. C. or under a reduced pressure of not more than 75 torr, thereby reducing the water content. This process makes it possible to obtain highly dehydrated ionic liquids.